EA036426B1 - Duplex stainless steel and use thereof - Google Patents

Abstract

The present disclosure relates to a corrosion resistant duplex stainless steel (ferritic austenitic alloy) which is suitable for use in a plant for the production of urea; and uses thereof. The disclosure also relates to objects made of said duplex stainless steel. Furthermore, the present disclosure also relates to a method for the production of urea and to a plant for the production of urea comprising one or more parts made from said duplex stainless steel, and to a method of modifying an existing plant for the production of urea.

Description

Technology area

This description applies to corrosion resistant duplex stainless steel (austenitic ferritic alloy) suitable for use in a urea plant. The description also applies to products made of said duplex stainless steel and the use of duplex stainless steel. In addition, the present disclosure also relates to a method for the production of urea and a plant for the production of urea containing one or more parts made of said duplex stainless steel, and to a method for retrofitting an existing plant for the production of urea.

Background of the invention

Duplex stainless steel is a ferritic-austenitic alloy. Such alloys have a microstructure with ferrite and austenite phases. Basic sources in this regard include WO 95/00674 and US 7347903. The duplex stainless steels described therein have high corrosion resistance and therefore can be used, for example, in the very corrosive environment of a urea plant.

Carbamide and its production.

Urea (NH2CONH2) can be produced from ammonia and carbon dioxide at elevated temperatures (typically 150 to 250 ° C) and pressures (typically 12 to 40 MPa) in the urea synthesis section of a urea plant. It can be considered that in such a synthesis, two successive stages of the reaction occur. In the first stage, ammonium carbamate is formed, and in the next stage, the resulting ammonium carbamate is dehydrated to form carbamide. The first stage (i) is exothermic, and the second stage can be represented as an endothermic equilibrium reaction (ii) (i) 2NH ₃ + СО ₂ H2N-CO-ONH4 (ii) H2N-CO-ONH4 θ H2N-CO-NH2 + Η ₂ Ο

In a standard urea plant, the above reactions are carried out in a synthesis section to produce an aqueous solution containing urea. This solution is concentrated in one or more subsequent concentration sections to ultimately obtain the urea as a melt rather than a solution. Thereafter, the melt goes through one or more finishing steps such as prilling, granulating, tableting or compacting.

A frequently used method for producing urea according to the stripping (stripping) method is the carbon dioxide stripping (stripping) method, for example, described in Ullmann's Encyclopedia of Industrial Chemistry, Vol. A27, 1996, pp. 333-350. In this method, the synthesis section is followed by one or more regeneration sections. The synthesis section contains a reactor, a stripper, a condenser and preferably, but not necessarily, a scrubber, the operating pressure of which is in the range from 12 to 18 MPa, for example in the range from 13 to 16 MPa.

In the synthesis section, the urea solution leaving the urea reactor is fed to a stripping apparatus, in which a large amount of unreacted ammonia and carbon dioxide is separated from the urea aqueous solution.

Such a stripper may be a shell-and-tube heat exchanger in which the urea solution is fed to the top from the tube side, and carbon dioxide for use in urea synthesis is introduced to the bottom of the stripper. Steam for heating the solution is introduced from the side of the casing. The urea solution exits the heat exchanger at the bottom and the vapor phase exits the stripper at the top. The vapor exiting this stripper contains ammonia, carbon dioxide, inert gases and a small amount of water.

This steam is generally condensed in a falling film heat exchanger or in a flooded condenser, which may be of the horizontal type or of the vertical type. A horizontal flooded heat exchanger is described in Ullmann's Encyclopedia of Industrial Chemistry, Vol. A27, 1996, pp 333-350. The resulting solution, which contains condensed ammonia, carbon dioxide, water and urea, is recycled together with non-condensed ammonia, carbon dioxide and inert vapor.

The processing conditions are highly corrosive, in particular due to the hot and concentrated carbamate solution. To try to prevent corrosion, oxygen is introduced as a passivating agent into the urea production process, usually in the form of a passivating air stream, i.e. some of the oxygen together with the chromium in the steel will form a protective layer of chromium oxide on the surfaces of stainless steel equipment.

In the past, corrosion was a problem, which consisted in the fact that urea production equipment, although made of stainless steel, and despite the addition of passivating air, corroded rather quickly, it had to be replaced prematurely, and also because that the presence of oxygen inevitably creates an unsafe situation. The problem was solved, in particular, through the manufacture of equipment, i.e. its corresponding parts exposed to said corrosive conditions of duplex stainless steel, in particular so-called super duplex stainless steel, as described in WO 95/00674 (which is sold under the trademark Safurex®). Such a super duplex stainless

- 1 036426 steel has a higher chromium content, as the combination of oxygen and duplex steel has resulted in a significant reduction in the amount of oxygen required for passivation and a lower level of passive corrosion. Thus, super duplex stainless steels that are used in carbamate environments such as urea plants exhibit very good performance but at high temperatures, i.e. in areas where the temperature is above 200 ° C, for example at 205 ° C, the level of passive corrosion can be higher than desired. Therefore, there remains a need for more corrosion-resistant duplex stainless steel that will extend the life of special equipment in a urea plant operating at higher temperatures, such as a HP (high pressure) stripper.

As will be apparent to a person skilled in the art, in general terms, the use of duplex steel in a carbamate environment involves the action of said carbamate on said duplex steel. Therefore, this use involves contact of the duplex steel with a fluid containing carbamate, such as a carbamate solution. This applies in particular to a concentrated carbamate solution, for example an ammonium carbamate solution with a concentration of 15 to 95% by weight of ammonium carbamate, for example 45 to 95% by weight. In particular, the carbamate-containing fluid has a high temperature, for example over 180 °, for example over 200 ° C.

In addition, another problem in the use of duplex stainless steels is associated with the fact that the initial microstructure, i.e. The microstructure that was inherent in duplex stainless steel after it was produced by the steelmaker may change after further processing of the steel, such as welding. The microstructural stability of duplex stainless steel is composition dependent, and when making complex parts, it is important to have a material whose microstructure is stable during processing in order to ensure adequate corrosion resistance as well as sufficient mechanical properties. Therefore, there is also a need for duplex stainless steel having a stable microstructure.

Thus, there remains a need for further improvements in duplex stainless steel materials used in urea plants, in particular for parts exposed to high temperatures and corrosive fluids, such as stripper tubes.

Therefore, it is desirable to propose a corrosion-resistant material characterized by a reduced rate of passive corrosion, in particular when exposed to fluids containing carbamate at high temperatures, such as, for example, in stripper tubes, in order to thereby prolong the life of the stripper tubes and at the same time to provide a sufficiently good structural stability of the stripper materials and, in particular, the structural stability of the heat-exposed zones in the welds connecting the heat exchanger tubes to the tube sheet.

Brief description of the invention

To meet one or more of the foregoing needs, in one aspect, the present disclosure provides a duplex stainless steel that contains, by weight%:

With max. 0.030;

Si max. 0.8;

Mn max. 2.0;

Cr 29.0 to 31.0;

Ni 5.0 to 9.0;

Mo less than 4.0;

W less than 4.0;

N 0.25-0.45;

Cu max. 2.0;

S max. 0.02;

P max. 0.03;

the rest is Fe and inevitably present impurities; and while the content of Mo + W is more than 3.0, but less than 4.0.

In the present description, the terms carbamate and ammonium carbamate are used interchangeably. The preferred carbamate is ammonium carbamate.

In addition, the present disclosure relates to the use of duplex stainless steel in a carbamate environment, whereby such duplex stainless steel contains, wt%:

With max. 0.030;

Si max. 0.8;

Mn max. 2.0;

Cr 29.0 to 31.0;

Ni 5.0 to 9.0;

Mo less than 5.0;

W less than 5.0;

- 2 036426

N 0.25 to 0.45;

Cu max. 2.0;

S max. 0.02;

P max. 0.03;

the rest is Fe and inevitably present impurities; and while the content of Mo + W is more than 3.0, but less than 4.0.

Moreover, the present disclosure relates to molded articles of duplex stainless steel described above or below and to the use of stainless steel as defined above or below in this document in a urea plant.

The present disclosure also relates to a process for the production of urea, wherein at least one piece of equipment is made of duplex stainless steel as defined above in or below in this document, and to a plant for the production of urea comprising one or more parts containing duplex stainless steel as defined above or below in this document.

In addition, the present disclosure also provides a method for modifying an existing urea plant and a method for reducing the passive corrosion rate of a urea plant by using duplex stainless steel as defined above or below in this document.

Detailed description

This description applies to duplex stainless steel, which contains, wt%:

With max. 0.030;

Si max. 0.8;

Mn max. 2.0;

Cr 29.0 to 31.0;

Ni 5.0 to 9.0;

Mo less than 4.0;

W less than 4.0;

N 0.25-0.45;

Cu max. 2.0;

S max. 0.02;

P max. 0.03;

the rest is Fe and inevitably present impurities; and while the content of Mo + W is more than 3.0, but less than 4.0.

Thus, for example, the present disclosure relates to duplex stainless steel which contains, by weight%:

With max. 0.020;

Si max. 0.8;

Mn max. 2.0;

Cr 29.0 to 31.0;

Ni 5.0 to 9.0;

Mo less than 4.0;

W less than 4.0;

N 0.25-0.45;

Cu max. 2.0;

S max. 0.01;

P max. 0.02;

the rest is Fe and inevitably present impurities; and while the content of Mo + W is more than 3.0, but less than 4.0.

In a broad sense, the present disclosure is based on the reasonable understanding that even better corrosion resistance is achieved by using duplex stainless steel as defined above or below in this document for those zones that are exposed to carbamate at high pressure and high temperature. Therefore, said duplex stainless steel is especially suitable for the manufacture of parts that are exposed to concentrated ammonium carbamate at high temperatures (greater than about 180 ° C), for example, parts for heat exchanger tubes and / or, or for example tubes in strippers. As will be obvious to the person skilled in the art, in particular in the case of exposure of equipment in a urea plant to carbamate, a typical concentrated carbamate solution has a concentration of 15 to 95 wt% ammonium carbamate, preferably 45 to 95 wt% and more preferably from 47 to 92 wt.%. In one embodiment, such an ammonium carbamate stream contains 85 to 92 wt% ammonium carbamate, such as present in the high pressure carbamate condenser of a urea plant. In another embodiment, such an ammonium carbamate stream contains 45 to 65 wt% ammonium carbamate, for example present in a pool condenser of a urea plant and / or at the inlet to a stripper, such as a high pressure stripper of a urea plant. , in particular installations with stripping

CO2.

Even though super duplex stainless steel, as described in WO 95/00674, has excellent corrosion resistance in carbamate solutions (even in the absence of oxygen) up to temperatures above 180 ° C, the passive corrosion rate of duplex stainless steel can be improved, in particular , at temperatures above approximately 180 ° C (recorded in the stripper tubes). Duplex stainless steel, as defined above or below in this document, exhibits markedly lower passive corrosion rates at these extreme temperatures. One of the advantages of duplex stainless steel is that it provides a longer expected life for the stripper, in particular the heat exchanger tubes.

This disclosure also relates to use in a carbamate environment, for example ammonium carbamate, duplex stainless steel as defined above or below, wherein the duplex stainless steel preferably contains, in percent, wt%:

With max. 0.030;

Si max. 0.8;

Mn max. 2.0;

Cr 29.0 to 31.0;

Ni 5.0 to 9.0;

Mo less than 5.0;

W less than 5.0;

N 0.25-0.45;

Cu max. 2.0;

S max. 0.02;

P max. 0.03;

the rest is Fe and inevitably present impurities; and while the content of Mo + W is more than 3.0, but less than 4.0.

Therefore, for example, the present disclosure relates to use in a carbamate environment, such as ammonium carbamate, duplex stainless steel as defined above or below, wherein the duplex stainless steel contains, wt%:

With max. 0.020;

Si max. 0.8;

Mn max. 2.0;

Cr 29.0 to 31.0;

Ni 5.0 to 9.0;

Mo less than 5.0;

W less than 5.0;

N 0.25-0.45;

Cu max. 2.0;

S max. 0.01;

P max. 0.02;

the rest is Fe and inevitably present impurities; and while the content of Mo + W is more than 3.0, but less than 5.0.

The inventors have surprisingly found that in the case of making stripper tubes from duplex stainless steel as defined above or below in this document, the oxygen addition to the process can be reduced to almost zero while maintaining the passive corrosion rate, which will be low in all parts of the plant. production of urea, as well as in the tubes of the stripper. Moreover, the inventors also found that commonly used tests for assessing corrosion of stainless steel (for example, the Streicher test with a test solution of ferrous sulfate and sulfuric acid, which is carried out at 127 ° C), which were used in the development of duplex stainless steel (as described in WO 95/00674) do not correlate with the actually observed corrosion of the particular equipment (stripper tube) in the urea plant. Therefore, further improvement of the passive corrosion rate of duplex stainless steel was possible only through the results of corrosion testing in a high pressure autoclave, which simulates the real process conditions that exist in specific equipment, for example, in stripper tubes.

The elemental composition of duplex stainless steel is generally as defined above or below in this document, and the functional characteristics of each alloy element are further described below.

Carbon (C) in the present description should be considered as an impurity element, and it has limited solubility in both the ferrite and austenite phase. This limited solubility suggests that, at too high percentages, there is a risk of carbide precipitation, as a result of which corrosion resistance will be reduced. Therefore, the C content should be limited to a maximum value of 0.030 wt%, for example a maximum value

0.020 wt%, for example a maximum value of 0.017 wt%, for example a maximum value of 0.015 wt%, for example a maximum value of 0.010 wt%.

Silicon (Si) is used in steelmaking as a deoxidation additive. However, too high a Si content increases the tendency for precipitation of intermetallic phases and a decrease in the N solubility. For this reason, the Si content should be limited to a max. 0.8 wt%, for example max. 0.5 wt%, for example in the range of 0.05 to 0.50 wt%, for example 0.1 to 0.5 wt%.

Manganese (Mn) is added to increase the solubility of N and to replace Ni as an alloy element, since Mn is considered an austenite stabilizing element. However, Mn can have a negative impact on structural stability, and therefore its content is max. 2.0 wt%, for example max. 1.5%, for example in the range from 0.5 to 1.5 wt%.

Chromium (Cr) is the most active element in terms of obtaining resistance to most types of corrosion. In the synthesis of urea, the Cr content is of great importance from the point of view of corrosion resistance, therefore, it should be as high as possible. However, there is a balance between high chromium content and good structural stability. Therefore, in the present specification, the Cr content should be in the range of 29.0 to 31.0 wt% to achieve sufficient corrosion resistance as well as structural stability. Therefore, the Cr content is 29.0 to 31.0 wt%, for example 29.00 to 30.00 wt%.

Nickel (Ni) is mainly used as an austenite stabilizing element. The advantage of using Ni is that it does not adversely affect structural stability. In order to ensure structural stability, the Ni content must be at least 5.0 wt%, since if the Ni content is lower than 5 wt%, chromium nitrides may be formed during the heat treatment. However, Ni can form a strong complex with ammonium, so the Ni content should be as low as possible. Thus, the Ni content is in the range of 5.0-9.0 wt%, for example 5.5-8.5 wt%, for example 5.5 to 7.5 wt%.

Molybdenum (Mo) is used to increase the passivity of duplex stainless steel. However, too high a Mo content is associated with the risk of precipitation of intermetallic phases. Therefore, the Mo content is less than 5.0 wt%, for example less than 4.0 wt%. Tungsten (W) improves pitting and crevice corrosion resistance. However, too high a W content increases the risk of precipitation of intermetallic phases, in particular in combination with high Cr and Mo contents. Therefore, the W content is less than 5.0 wt%, for example less than 4.0 wt%. To achieve the highest possible anticorrosive properties, the Mo + W content should be as high as possible, while the sensitivity to the sigma phase should not be unreasonably high. If the Mo + W content exceeds 5.0 wt%, the determinant of sigma phase formation will be so high that it will be difficult to obtain components without sigma phase. However, in accordance with the present disclosure, it has been shown that if W + Mo is greater than 3.0 wt%, the duplex stainless steel will exhibit an even lower level of corrosion in the stripper tube. Therefore, the Mo + W content is more than 3.0 wt% but less than 5.0 wt%, for example less than 4.0 wt%. Moreover, if the W + Mo content exceeds 3.0 wt% but less than 4.0 wt%, the duplex stainless steel as defined above or below in this document contains a small amount of a sigma phase, for example, essentially does not contain sigma phase, e.g. max. 0.5 wt%, for example max. 0.05 wt%. Preferably, the formation of a sigma phase should be avoided as it can cause embrittlement of duplex stainless steel and thus reduce corrosion resistance.

Nitrogen (N) is an effective austenite-forming element and enhances the dissolution of austenite. In addition, N influences the distribution of Cr, Mo and Ni in the austenite phase and ferrite phase. Therefore, a higher N content increases the relative proportion of Cr and Mo in the austenite phase. This means that austenite becomes more resistant to corrosion and that more Cr and Mo can be added to duplex stainless steels while maintaining structural stability. Therefore, the N content should be at least 0.25% by weight. However, the solubility of nitrogen is limited and too high a nitrogen level will increase the risk of chromium nitride formation, which in turn will affect corrosion resistance. Therefore, the N content should not exceed 0.45% by weight. Thus, the N content is 0.25 to 0.45 wt%, for example 0.28 to 0.40 wt%.

Copper (Cu) is an optional element in the present specification and, when added, enhances the overall corrosion resistance in acidic environments such as sulfuric acid. However, a high Cu content decreases the pitting and crevice corrosion resistance. Therefore, the Cu content should be limited to max. 2.0 wt%, for example max. 1.0 wt%, for example max. 0.8 wt%.

Sulfur (S) negatively affects corrosion resistance by forming readily soluble sulfides. Therefore, the S content should be limited to max. 0.02 wt%, for example max. 0.01 wt%.

Phosphorus (P) is a common impurity element. If present in amounts greater than about 0.03% by weight, it can have adverse effects such as hot ductility, weldability and corrosion resistance. The P content of the alloy should be limited to max. 0.03 wt%, for example max. 0.02 wt%.

When the term max (maximum value) is used, one skilled in the art will understand that the lower end of the range is 0 wt% unless otherwise indicated. Thus, the lower limit for C, Si, Mn, Cu, S and P is 0 wt%, since they are optional components.

In addition, if necessary, other elements can be added to duplex stainless steel as defined above or below in this document in the manufacturing process to improve workability, such as hot workability, machinability, and the like. Non-limiting examples of such elements include Ti, Nb, Hf, Ca, Al, Ba, V, Ce and B. If added, such elements are added in amounts totaling max. 0.5 wt%. For example, if necessary, it is possible for an alloy as defined above or below in this document, which contains the specified elements C, Si, Mn, Cr, Ni, Mo, W, N, Cu, S and P in the specified amounts and in which the rest falls on Fe + inevitable impurities, contained the mentioned specified elements in the mentioned amounts plus max. 0.5 wt.% Added optional elements, for example added for workability, such as Ti, Nb, Hf, Ca, Al, Ba, V, Ce and B, with the rest being Fe + unavoidable impurities.

The remainder in duplex stainless steel as defined above or below in this document is Fe and unavoidable impurities. Examples of unavoidable impurities include elements and compounds that are not intentionally added but cannot be completely avoided since they are usually present as impurities, for example, in the starting material used to make duplex stainless steel.

The ferrite content of duplex stainless steel as described herein is important from a corrosion resistance point of view. Therefore, the ferrite content is preferably in the range of 30 to 70 vol%, for example in the range of 30 to 60 vol%, for example in the range of 30 to 55 vol%, for example in the range of 40 to 60 vol%.

Duplex stainless steel, as defined above or below in this document, can be manufactured in accordance with conventional methods, for example, casting followed by hot working and / or cold working and optionally additional heat treatment. Duplex stainless steel, as defined above or below in this document, can also be obtained in the form of a powder product, for example, for the hot isostatic pressing (HIP) process.

Duplex stainless steel as defined above or below in this document may have other applications where high corrosion resistance is required for equipment. Some examples of possible uses for duplex stainless steel include as structural materials for chemical process components that are intended for use in nitric acid, in melamine production, in the pulp and paper industry, such as white liquor, and as a welding electrode material. wire. Steel can be used, for example, for the production of seamless pipes, welded pipes, flanges, couplings and sheet metal.

The present disclosure also relates to a molded article comprising duplex stainless steel, according to one embodiment, said article is a tube, for example a stripper tube for a urea plant or a liquid distributor for a stripper in a urea plant. The present disclosure also relates to the use of duplex stainless steel, as defined above or below in this document, in any of the embodiments described above and below in this document, in the process of synthesizing urea. Such use of duplex stainless steel as defined above or below in this document is intended to reduce corrosion of one or more parts of equipment used in the process, for example one or more parts of the high pressure urea synthesis section, for example parts that come into contact with a carbamate solution ...

Another aspect of the present disclosure relates to the provision of a method for producing urea, in which at least one of the equipment parts, for example, the part that comes into contact with the carbamate solution, is made of duplex stainless steel as defined above or below in this document. The oxygen content of the carbamate solution may be less than 0.1 ppm, for example less than 0.04 ppm (by weight).

Another aspect of the present disclosure relates to the provision of a plant for the production of urea, said plant including one or more parts comprising duplex stainless steel as defined above or below in this document. In accordance with one embodiment, one or more tubes of the stripper comprises or is made of duplex stainless steel as defined above or below herein. In accordance with another embodiment, the plant includes a high-grade urea synthesis section.

- 6 036426 pressure, which contains a stripper, and the stripper includes at least one liquid distributor containing duplex stainless steel, as defined above or below in this document. Said duplex stainless steel can be used in a method for retrofitting an existing urea plant, said plant comprising one or more components selected from the group consisting of liquid distributors, radar cones, (control) valves and ejectors, said method being characterized in that that one or more stripper tubes are replaced by a stripper tube comprising duplex stainless steel as defined above or below herein. The method can also be applied to a method of reducing the corrosion rate of a urea plant by replacing at least one stripper tube with a stripper tube containing duplex stainless steel as defined above or below herein.

The present description also includes the following non-limiting embodiments:

Embodiment 1.0. The use of duplex stainless steel in a carbamate environment, while such duplex stainless steel contains, wt%:

With max. 0.030;

Si max. 0.8;

Mn max. 2.0;

Cr 29.0 to 31.0;

Ni 5.0 to 9.0;

Mo less than 4.0;

W less than 4.0;

N 0.25-0.45;

Cu max. 2.0;

S max. 0.02;

P max. 0.03;

the rest is Fe and inevitably present impurities; and while the content of Mo + W is more than 3.0, but less than 4.0.

Embodiment 1.1. The use of duplex stainless steel in a carbamate environment, while such duplex stainless steel contains, wt%:

With max. 0.020;

Si max. 0.8;

Mn max. 2.0;

Cr 29.0 to 31.0;

Ni 5.0 to 9.0;

Mo less than 5.0;

W less than 5.0;

N 0.25-0.45;

Cu max. 2.0;

S max. 0.01;

P max. 0.02;

the rest is Fe and inevitably present impurities; and while the content of Mo + W is more than 3.0, but less than 5.0.

Embodiment 1.2. Use of duplex stainless steel according to Embodiment 1.0 or 1.1, wherein the Mn content is 0.5-1.5 wt%.

Embodiment 1.3. The use of a duplex stainless steel according to embodiment 1.0, 1.1 or 1.2, wherein the Si content is 0.010 to 0.50 wt%.

Embodiment 1.4. The use of duplex stainless steel according to any of the embodiments 1.0 to 1.3, wherein the Ni content is about 5.5 to 8.5 wt%, for example 5.5 to 7.5 wt%.

Embodiment 1.5. The use of duplex stainless steel according to any of the embodiments 1.0 to 1.4, wherein the N content is 0.28 to 0.40 wt%.

Embodiment 1.6. The use of duplex stainless steel in accordance with any of the embodiments 1.0 to 1.5, wherein the duplex stainless steel contains, wt%:

With max. 0.030;

Si max. 0.8;

Mn max. 2.0;

Cr 29.0 to 31.0;

Ni 5.0 to 9.0;

Mo less than 4.0;

W less than 4.0;

N 0.25-0.45;

- 7 036426

Cu max. 2.0;

S max. 0.02;

P max. 0.03.

Embodiment 1.7. The use of duplex stainless steel in accordance with embodiment 1.6, wherein the duplex stainless steel contains, wt. %:

With max. 0.020;

Si max. 0.8;

Mn max. 2.0;

Cr 29.0 to 31.0;

Ni 5.0 to 9.0;

Mo less than 4.0;

W less than 4.0;

N 0.25-0.45;

Cu max. 2.0;

S max. 0.01;

P max. 0.02;

the rest is Fe and inevitably present impurities; and the content of Mo + W is more than 3.0, but less than 4.0.

Embodiment 1.8. Use of duplex stainless steel according to any one of Embodiments 1.0-1.7 in a urea synthesis process to reduce corrosion of one or more high pressure urea synthesis section parts that come into contact with an ammonium carbamate solution.

Embodiment 1.9. A molded product comprising duplex stainless steel as described in any one of Embodiments 1.0-1.7, said molded product being a pipe, a stripper tube for a urea plant, or a liquid distributor for a stripper for a urea plant.

Embodiment 1.10. A method for producing urea, wherein at least one piece of equipment is made of duplex stainless steel as defined in any one of Embodiments 1.0-1.7, and such a method preferably comprises the formation of ammonium carbamate and the dehydration of the ammonium carbamate to produce urea.

Embodiment 1.11. A plant for the production of urea, wherein said plant includes one or more parts comprising duplex stainless steel as defined in any one of Embodiments 1.0-1.7.

Embodiment 1.12. An apparatus according to embodiment 1.11, wherein one or more of the parts are one or more tubes of the stripper.

Embodiment 1.13. An apparatus in accordance with Embodiments 1.11 or 1.12, including a high pressure urea synthesis section containing a stripper, wherein the stripper includes at least one liquid distributor containing duplex stainless steel as defined in any one of Embodiments 1.0 -1.7.

Embodiment 1.14. A method for modifying an existing urea plant, said plant comprising one or more components selected from the group consisting of liquid distributors, radar cones, (control) valves and ejectors, wherein said method is characterized in that one or more stripper tubes is replaced by a stripper tube comprising duplex stainless steel as defined in any of Embodiments 1.0-1.7.

Embodiment 1.15. A method of reducing the passive corrosion rate of a urea plant by replacing at least one stripper tube with a stripper tube containing duplex stainless steel as defined in any one of Embodiments 1.0-1.7.

Embodiment 2.0 Duplex stainless steel, which contains, weight. %:

With max. 0.030;

Si max. 0.8;

Mn max. 2.0;

Cr 29.0 to 31.0;

Ni 5.0 to 9.0;

Mo is less than 4.0;

W less than 4.0;

N 0.25-0.45;

Cu max. 2.0;

S max. 0.02;

P max. 0.03;

the rest is Fe and inevitably present impurities; and

- 8 036426 while the Mo + W content is more than 3.0, but less than 4.0.

Embodiment 2.1. Duplex stainless steel, which contains, wt%:

With max. 0.020;

Si max. 0.8;

Mn max. 2.0;

Cr 29.0 to 31.0;

Ni 5.0 to 9.0;

Mo less than 4.0;

W less than 4.0;

N 0.25-0.45;

Cu max. 2.0;

S max. 0.01;

P max. 0.02;

the rest is Fe and inevitably present impurities; and while the content of Mo + W is more than 3.0, but less than 4.0.

Embodiment 2.2. Duplex stainless steel according to embodiment 2.0 or 2.1, wherein the Mn content is 0.5-1.5% by weight.

Embodiment 2.3. Duplex stainless steel according to embodiment 2.0, 2.1, or 2.2, wherein the Si content is 0.010 to 0.50 wt%.

Embodiment 2.4. Duplex stainless steel according to any of embodiments 2.0 to 2.3, wherein the Ni content is 5.5 to 8.5 wt%, for example 5.5 to 7.5 wt%.

Embodiment 2.5. Duplex stainless steel according to any of embodiments 2.0 to 2.4, wherein the N content is 0.28 to 0.40 wt%.

Embodiment 2.6. Duplex stainless steel according to embodiment 2.0 or 2.1, wherein the Mn content is 0.5-1.5 wt%, the Si content is 0.010 to 0.50 wt%, the Ni content is 5.5 to 8 , 5 wt.% And the N content is from 0.28 to 0.40 wt.%.

Embodiment 2.7. A molded article comprising duplex stainless steel in accordance with any of embodiments 2.0 to 2.6.

The present description is further illustrated by the following non-limiting examples.

Examples of

Table 1 shows the compositions of duplex stainless steels used in the examples. The parts used for testing were made from 270 kg billets hot forged, hot rolled, cold rolled and then heat treated.

Corrosion tests using autoclaves.

Samples were cut from 5 mm strips that were hot worked to about 1200 ° C and cold rolled (room temperature) with intermediate (about 1100 ° C) and final annealing at 1070 ° C. The samples that were used for testing were in the form of bars with an approximate size of 20x10x3 mm. All surfaces were treated and finished with wet grinding.

The corrosion resistance of duplex stainless steel was evaluated in an oxygen-free carbamate solution. The composition of the carbamate solution was chosen to simulate even more severe conditions than would normally exist in the heat exchanger tubes of a stripper in a urea plant. The temperature during the tests was 210 ° C. The corrosion rate was calculated after exposure to an oxygen-free carbamate solution for 14 days. The results are presented in table. 3. As can be seen from the table, Charges 1 and 2 exhibit higher corrosion resistance than Comparative Charges 3-5, as indicated by a lower corrosion rate.

The procedure below was used to investigate the effect. The autoclave was thoroughly washed and cleaned with ultrapure water and ethanol. Samples (stripes) were purified in acetone and ethanol and weighed, and the stripe sizes were measured. Then they were installed in a Teflon sample holder.

Water and urea were added to the autoclave. The autoclave was then purged with nitrogen to remove oxygen and other gases. Thereafter, ammonia was added to the autoclave.

The next day, heating was started in accordance with the temperature profile given in table. 2. The sequence was built in such a way as to exclude overheating. The samples were kept at 210 ° C for 14 days.

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Table 1

Composition of downloads for examples

Loading FROM Si Mp R S Cr Ni Moe W Mo + W N Si one 0.012 0.08 one 0.008 0.008 29.07 5.76 0.48 2.55 3.03 0.35 0.01 2 0.012 0.23 1.05 0.005 0.005 29.92 7.17 3.01 - 3.01 0.3 - 3 0.011 0.48 1.06 0.004 0.006 28.74 6.84 2.24 - 2.24 0.34 <0.010 4 0.010 0.11 1.09 0.005 0.006 33.31 6.5 0.48 - 0.48 0.41 <0.010 5 0.010 0.48 4.07 0.004 0.007 30.77 5.08 0.33 - 0.33 0.33 <0.010

table 2

Autoclave heating sequence

Starting pace. (° C) End temp. (° C) Heating rate (° C / min)

one kt 195 one 2 195 208 0.2 3 208 210 0.1

Table 3

Loading Corrosion rate [mm / year] one 0.186 2 0.191 3 0.223 4 0.275 5 0.329

Mechanical tests.

The mechanical properties were evaluated by tensile tests, impact tests and hardness measurements. For tensile tests and hardness measurements, 5 mm cold rolled and annealed strips were used. Hot rolled 11 mm strips were used for shock testing. The production of strips is described above.

Tensile tests were conducted at room temperature in accordance with ISO6892-l: 2009.

The impact test pieces were V-notch test pieces (SSV1). The tests were carried out in accordance with ISO14556. The tests were carried out at two temperatures: at room temperature and at -35 ° C.

Hardness measurements were carried out on the cross-sectional surface of longitudinal specimens obtained from a 5 mm strip. The measurements were carried out in the center of the strip. Measurement of Vickers hardness with a load of 98.07 N (10 kg) (98.07 MPa (HV10)).

Measurements of the inter-austenite interval were carried out on the same samples as for measurements of hardness. Measurements were performed in accordance with Recommended Practice DNV-RP-F112 Section 7 (October 2008).

Mechanical test results are shown in the tables below.

Table 4A

Tensile test results

Loading Rp 0.2 (MPa) Rp 0.1 (MPa) Rm (MPa) A (%) one 626 717 865 thirty 2 648 744 878 27 3 566 669 831 31 4 669 755 883 28 5 617 703 817 28

Table 4B

Impact test results at room temperature

Loading CT-1 CT-2 CT-3 one 146 169 153 2 194 188 178 3 202 208 213 4 172 178 178 5 143 135 150

Table 4C

Impact test results at -35 ° C

Loading Test 1 (J) Test 2 (J) Test 3 (J) Average value (J) one 107 146 130 128 2 145 141 137 141 3 165 179 186 177 4 37 40 39 39 5 24 23 twenty 22

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Table 4D

Hardness test results

98.07 MPa (HV10) Loading Imprint 1 Imprint 2 Imprint 3 Average one 309 283 285 292 2 292 292 283 289 3 285 285 292 287 4 279 292 297 289 5 266 276 281 274

Table 4E

Results of measurements of the inter-austenite interval

Claims (16)

CLAIM 1. Application of duplex stainless steel, which contains, wt%: With max. 0.030; Si max. 0.8; Mn max. 2.0; Cr 29.0 to 31.0; Ni 5.0 to 9.0; Mo less than 4.0; W less than 4.0; N 0.25-0.45; Cu max. 2.0; S max. 0.02; P max. 0.03; Ti, Nb, Hf, Ca, Al, Ba, V, Ce and B in total max. 0.5; the rest is Fe and inevitably present impurities; and a Mo + W content of more than 3.0 but less than 5.0 as a material of construction for the components of the urea plant in contact with the carbamate solution. 2. The use of duplex stainless steel according to claim 1, wherein the duplex stainless steel contains, wt%: With max. 0.020; Si max. 0.8; Mn max. 2.0; Cr 29.0 to 31.0; Ni 5.0 to 9.0; Mo less than 4.0; W less than 4.0; N 0.25 to 0.45; Cu max. 2.0; S max. 0.01; P max. 0.02; Ti, Nb, Hf, Ca, Al, Ba, V, Ce and B in total maximum 0.5; the rest is Fe and inevitably present impurities; and the content of Mo + W is more than 3.0, but less than 5.0. 3. Use of duplex stainless steel according to claim 1, wherein the content in wt% is Mo is less than 4.0; W less than 4.0; and while the content of Mo + W is more than 3.0, but less than 4.0. 4. Use of a duplex stainless steel according to any one of claims 1 to 3, wherein the Mn content is 0.5-1.5 wt%. 5. The use of duplex stainless steel according to any one of claims 1 to 4, wherein the Si content is from 0.010 to 0.50 wt%. 6. Use of duplex stainless steel according to any one of claims 1 to 5, wherein the Ni content is from 5.5 to 8.5 wt%. - 11 036426 7. Use of duplex stainless steel according to any one of claims 1 to 6, wherein the N content is 0.28 to 0.40% by weight. 8. Use of a duplex stainless steel according to any one of claims 1 to 7, wherein the Mn content is 0.5-1.5 wt%, the Si content is 0.010 to 0.50 wt%, the Ni content is 5.5 to 8.5 wt% and the N content is 0.28 to 0.40 wt%. 9. The use of duplex stainless steel according to any one of claims 1-8 in the urea synthesis process to reduce corrosion of one or more parts of the high pressure urea synthesis section that come into contact with the ammonium carbamate solution. 10. Molded product containing duplex stainless steel, which contains, wt%: With max. 0.030; Si max. 0.8; Mn max. 2.0; Cr 29.0 to 31.0; Ni 5.0 to 9.0; Mo less than 4.0; W less than 4.0; N 0.25-0.45; Cu max. 2.0; S max. 0.02; P max. 0.03; Ti, Nb, Hf, Ca, Al, Ba, V, Ce and B in total maximum 0.5; the rest is Fe and inevitably present impurities; and wherein the Mo + W content is greater than 3.0 but less than 5.0, said molded article being a stripper tube of a urea plant or a liquid distributor for a stripper of a urea plant. 11. A method for the production of urea, including the reaction of ammonia and carbon dioxide under the conditions of the formation of ammonium carbamate and the dehydration of ammonium carbamate to obtain urea in a urea plant, in which at least one piece of equipment for the urea production unit is made of duplex stainless steel, which contains , weight.%: With max. 0.030; Si max. 0.8; Mn max. 2.0; Cr 29.0 to 31.0; Ni 5.0 to 9.0; Mo less than 4.0; W less than 4.0; N 0.25-0.45; Cu max. 2.0; S max. 0.02; P max. 0.03; Ti, Nb, Hf, Ca, Al, Ba, V, Ce and B in total maximum 0.5; the rest is Fe and inevitably present impurities; and while the content of Mo + W is more than 3.0, but less than 5.0. 12. Installation for the production of urea, which includes one or more parts, which are exposed to a fluid containing carbamate, and the specified one or more parts contain duplex stainless steel, which contains, wt.%: With max. 0.030; Si max. 0.8; Mn max. 2.0; Cr 29.0 to 31.0; Ni 5.0 to 9.0; Mo less than 4.0; W less than 4.0; N 0.25-0.45; Cu max. 2.0; S max. 0.02; P max. 0.03; Ti, Nb, Hf, Ca, Al, Ba, V, Ce and B in total maximum 0.5; the rest is Fe and inevitably present impurities; and while the content of Mo + W is more than 3.0, but less than 5.0. 13. An apparatus according to claim 12, wherein said one or more parts are one or more stripping apparatus tubes. - 12 036426 14. A plant according to claim 12 or 13, comprising a high-pressure urea synthesis section comprising a stripper, wherein the stripper comprises at least one liquid distributor containing duplex stainless steel. 15. A method for modifying a plant for the production of urea, which contains one or more components selected from the group consisting of liquid distributors, radar cones, (control) valves and ejectors, said method including the step of replacing one or more tubes of the stripper a stripper tube containing duplex stainless steel, which contains, wt%: With max. 0.030; Si max. 0.8; Mn max. 2.0; Cr 29.0 to 31.0; Ni 5.0 to 9.0; Mo less than 4.0; W less than 4.0; N 0.25-0.45; Cu max. 2.0; S max. 0.02; P max. 0.03; Ti, Nb, Hf, Ca, Al, Ba, V, Ce and B in total maximum 0.5; the rest is Fe and inevitably present impurities; and while the content of Mo + W is more than 3.0, but less than 5.0. 16. A method of reducing the rate of passive corrosion of a plant for the production of urea by replacing at least one tube of the stripper with a tube of the stripper containing duplex stainless steel, which contains, wt%: With max. 0.030; Si max. 0.8; Mn max. 2.0; Cr 29.0 to 31.0; Ni 5.0 to 9.0; Mo less than 4.0; W less than 4.0; N 0.25-0.45; Cu max. 2.0; S max. 0.02; P max. 0.03; Ti, Nb, Hf, Ca, Al, Ba, V, Ce and B in total maximum 0.5; the rest is Fe and inevitably present impurities; and while the content of Mo + W is more than 3.0, but less than 5.0.